US20120025112A1 - Device and method for detecting liquid level of molten silicon using laser reflected from curved mirror - Google Patents

Abstract

The present invention discloses a laser liquid level detection device with linear optical magnification of a curved minor, wherein two openings are disposed at a top of a single crystal furnace symmetrically, a laser device and a CCD detecting device are respectively disposed on the two openings, and signal processing devices are pre-disposed within the laser device and the CCD detecting device. In the liquid surface height detection method using the device of the present invention, a model is first established to simulate the laser propagating path, a polynomial curve function of the curved mirror satisfying the requirement is calculated, the prepared curved minor is then disposed in the CCD detecting device, the change in the height of the liquid surface of molten silicon may be reflected on a CCD sensor, and therefore the actual change in the height of the liquid surface of molten silicon may be calculated. When detecting a liquid surface using the laser liquid surface detection device of the present invention, the position of the liquid surface of the single crystal furnace can be detected in real time, the detection accuracy is improved, and the reading and writing of the signal is facilitated, with the advantages of high accuracy, short time for detection, low failure rate, and easy installation and operation.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)
• This application claims the benefit under 35 U.S.C. §119(a) of Chinese Patent Application No. 201010246742.4, filed on Jul. 30, 2010, in the State Intellectual Property Office of the People's Republic of China, the entire disclosure of which is incorporated herein by reference for all purposes.
FIELD OF THE INVENTION
• The present invention belongs to the technical field of detection technology and automatic equipment, and relates to a device for detecting a liquid level of melted silicon using a laser reflected from a curved mirror. The present invention also relates to a method for performing a liquid level detection of melted silicon using a laser reflected from a curved minor, for a liquid surface detection of a single crystal furnace.
DESCRIPTION OF RELATED ART
• Recently, during a liquid level detection of melted silicon using an oblique incidence and oblique reception type Laser Triangulation Method, due to the limitations of the special internal and external structures of the single crystal furnace and the requirement of the crystal pulling process, a distance between a position of a laser incident point and a photodetector is small, and a vertical distance between the horizontal plane where the laser incident point is located and the liquid surface is large. A laser is incident from the incident point, enters into the photodetector after being reflected from a liquid surface of molten silicon, and then is reflected to a sensor by a flat mirror within the photodetector. When the liquid level of the liquid surface of molten silicon is slightly changed, the change in the displacement of a spot, which is displayed on the sensor after the laser is reflected from the flat minor within the photodetector, is identical with the change in the height of the liquid level, such that the change in the displacement of the spot reflected onto the sensor is small, causing a low resolution of the liquid level detection and a low detection accuracy.
• In a light path of a laser triangulation liquid level detection system, if a flat minor is used to change a light path of a light reflected by the liquid surface, it is impossible to magnify the slight change in the liquid level as the change in the displacement of the spot on the detector; and if a cylindrical mirror is used to change the path of the light reflected by the liquid surface, it is possible to magnify the slight change in the liquid level as the change in the displacement of the spot on the sensor, but such magnification is nonlinear, and thus the change in the spot position cannot directly reflect the change in the position of the liquid surface.
SUMMARY OF THE INVENTION
• An object of the present invention is to provide a device for detecting a liquid level of melted silicon using a laser reflected from a curved mirror with improved accuracy of detecting the liquid surface of melted silicon.
• Another object of the present invention is to provide a method of detecting a liquid level of melted silicon using a laser reflected from a curved minor.
• A technical solution adopted by the present invention is a device for detecting a liquid level of melted silicon using a laser reflected from a curved mirror, comprises two openings disposed at a top of a single crystal furnace symmetrically, gold-plated quartz glasses disposed at the two openings, a laser device and a CCD detecting device respectively disposed corresponding to the two openings, and signal processing devices disposed within the laser device and the CCD detecting device;
• the CCD detecting device has a structure comprising: a detecting device housing, an optical filter horizontally disposed at a bottom of the detecting device housing, a driving board vertically disposed at an inner wall of the detecting device housing, a CCD sensor vertically disposed at an inner side of the driving board, a curved minor having a concave reflective surface disposed within the detecting device housing, and a connector disposed at a top of the detecting device housing, wherein the reflective surface of the curved mirror is disposed obliquely, the reflective surface faces the optical filter and the CCD sensor;
• the laser device has a structure comprising: a laser device housing, a laser disposed within the laser device housing, and a temperature control device disposed at an outer surface of the laser, a control circuit board to which the laser and the temperature control device are respectively connected, and an electrical connector and a cooling air port respectively disposed at a top of the laser device housing;
• the signal processing device has a structure comprising: a laser driving circuit, a laser temperature control circuit, and a microprocessor a disposed within the laser device, and a CCD driving circuit, an image signal processing circuit, and a microprocessor b disposed within the laser device, wherein the laser device and the CCD detecting device are respectively connected to an RS485 bus.
• Another technical solution adopted by the present invention is a method for detecting a liquid level of melted silicon using a laser reflected from a curved mirror, comprising the specific steps of:
• Step 1
• disposing two openings at a top of a single crystal furnace symmetrically, installing gold-plated quartz glasses installed at the two openings, disposing a laser device and a CCD detecting device in which a curved mirror is not installed respectively disposed at positions corresponding to the two openings, wherein signal processing devices are pre-disposed within the laser device and the CCD detecting device,
• wherein the signal processing device has a structure comprising: a laser driving circuit, a laser temperature control circuit, and a microprocessor a disposed within the laser device, and a CCD driving circuit, an image signal processing circuit, and a microprocessor b disposed within the laser device, wherein the laser device and the CCD detecting device are respectively connected to an RS485 bus;
• Step 2
• determining a shape of a reflective surface of the curved minor and a position of the curved minor within the CCD detecting device, which comprising the specific processes as follows
• a laser path formed by laser incident from an incident point of the laser device is mapped to a corresponding two-dimensional coordinate system, it is assumed that the curved mirror is disposed within the CCD detecting device, a reflective surface of the curved minor is disposed obliquely, the reflective surface faces an optical filer and a CCD sensor, and the laser is reflected by the curved mirror after being reflected from a liquid surface of melted silicon, in the established coordinate system, the laser path is analyzed, wherein the laser is emitted from a laser incident point, a horizontal plane between the laser incident point and a bottom of the CCD detecting device is set as an x-axis, a coordinate of the laser incident point is set as A(−m₀,0), an angle formed by the laser incident point and the x-axis is set as β, when the emitted laser reaches a lowermost end of the reflective surface of the curved minor after being reflected from the liquid surface of melted silicon liquid, an intersection point of the laser reflected from the liquid surface of melted silicon and the x-axis is set as A₁(m₀,0), an axis vertically passing through a midpoint of A(−m₀,0) and A₁(m₀,0) is set as a y-axis, a current distance between the x-axis and the liquid surface of melted silicon is set as d₀, and a current height of the liquid surface of the melted silicon is a maximum measurable value; when the height of the liquid surface of melted silicon decreases and the emitted laser reaches an uppermost end of the reflective surface of the curved mirror after being reflected from the liquid surface of melted silicon, a current height of the liquid surface of melted silicon is a minimum measurable value, and a difference between the maximum value and the minimum value of the height of the liquid surface of melted silicon is the detectable range r; when the height of the liquid surface of molten silicon is changed from high to low, reflecting points of the laser on the liquid surface of melted silicon are set as V₀(T₀,D₀), V₁(T₁,D₁) . . . , V_i(T_i,D_i) in sequence and the lines formed by the reflection of the liquid surface of melted silicon are set as f₀, f₁, . . . , f_iin sequence, a curve function of the reflective surface of the curved mirror is set as y=f(x), reflecting points on the curved minor are set as C₀(x₀,y₀), C₁(x₁,y₁), . . . , C_i(x_i,y_i) in sequence, lines formed by the reflection of the curved mirror are set as f₀′, f₁′, . . . , f_i′ in sequence, angles formed by f₀′, f₁′, . . . , f_i′ and the x-axis are set as α₀, α₁, . . . , α_iin sequence, and a reflection tangent line of f_iand f_i′ at the point C_i(x_i,y_i) is set as l_i, the laser is finally incident to the CCD sensor which is disposed vertically, a line representing the CCD sensor is x=L+m₀, and incident points on the CCD sensor are set as P₀(L+m₀,Q₀), P₁(L+m₀,Q₁), . . . , P_i(L+m₀,Q_i) in sequence,
• calculating discrete points C_i(x_i,y_i) on the curve of the curved mirror, which comprising
• first l₀is obtained using f₀and f₀′ according to the optical reflection principle, wherein V₀(T₀,D₀) on l₀and an angle β are known, therefore the function of f₀is known, and C₀(x₀,y₀) and P₀(L+m₀,Q₀) are determined; the reflection tangent line l₀at the intersection point C₀(x₀,y₀) is obtained according to the incident line f₀and the reflected line f₀′; C₁(x₁,y₁) is obtained through C₀(x₀,y₀) by calculating the point P₁(L+m₀,Q₁) on the CCD sensor when the liquid surface moves downwards by a distance d, that is, the reflecting point of the liquid surface is changed to be V₁(T₁,D₁), the point P₁(L+m₀,Q₁) on the CCD sensor is determined, the intersection point C₁(x₁,y₁) of l₀and f₁is obtained, the line f₁′ is obtained using the points C₁(x₁,y₁) and P₁(L+m₀,Q₁), and then the reflection tangent line l₁at the intersection point C₁(x₁,y₁) of lines f₁and f₁′ is obtained using lines f₁and f₁′ according to the optical reflection principle, so that C₁(x₁,y₁) is calculated based on C₀(x₀,y₀); the line f_i′ is obtained using the points C_i(x_i,y_i), which is an intersection point of l_i-1and f_i, and P_i(L+m₀,Q_i), and then the line l_iis obtained using f_iand f_i′ according to the optical reflection principle, using iterative method until all the discrete data points C_i(x_i,y_i) are obtained, wherein i=0, 1, 2, . . . , n,
• polynomial curve fitting using the obtained discrete data points C_i(x_i,y_i), which comprising
• an M-order polynomial curve fitting is performed using the discrete data points C_i(x_i,y_i) obtained by the above calculation, wherein the fitted polynomial curve of the curved mirror is as follows:
• 
  y=f(x)=α_Mx^M+α_M-1x^M-1+ . . . +α₀
• wherein x is a value selected from a range (x₀,x_n), M is a value selected from a range 3-5, and α₀, α₁, . . . , α_Mare the parameters given by the least squares curve fitting algorithm;
• Step 3
• preparing the curved mirror according to the polynomial curve function of the curved minor calculated instep 2, such that a radian function of the reflective surface of the curved mirror is consistent with the polynomial curve function of the curved mirror, next, disposing the curved minor within the CCD detecting device, such that the reflective surface of the curved minor is disposed obliquely, and the reflective surface faces the optical filter and the CCD sensor, wherein the position of the curved mirror within the CCD detecting device is determined by the values defined by the polynomial curve function of the curved minor obtained as above;
• Step 4
• detecting a change in the height of the liquid surface of melted silicon, comprising: activating the laser device, emitting the laser from the laser incident point of the laser device towards the liquid surface of melted silicon, detecting the change in the height of the liquid surface of melted silicon by the CCD sensor when the laser enters into the CCD detecting device after being reflecting from the liquid surface of melted silicon, wherein the maximum variable range r of the height of the melted silicon liquid surface is known and the maximum measureable range R of a spot on the CCD sensor is known, therefore, the magnification of thecurved mirror11 is obtained as N=R/r, the spot on the CCD sensor is changed as the liquid level of the liquid surface of melted silicon is changed, the changed value of the height of the spot on the CCD sensor is recorded, and then is divided by the magnification N, and therefore, the changed value of the height of the liquid surface of melted silicon is obtained.
• The advantageous technical effects of the present invention are as follows: the position of the liquid surface of the single crystal furnace can be detected in real time; when the liquid surface of melted silicon is slightly changed, a large displacement occurs between the incident position (after the laser beam is reflected by the concave surface of the curved mirror11) and the former incident position on the CCD sensor, such that the technical problem in the related art of a low optical resolution due to the structure of the conventional detecting device is solved, and therefore the detection accuracy is improved and the reading and writing of the signal is facilitated, with the advantages of high accuracy, short time for detection, low failure rate, and easy installation and operation. A curved minor is provided such that a slight change in a liquid level is linearly magnified as a change in a displacement of a spot on a detector, while a numerical algorithm for the function of the curved mirror is given. The detecting method may also be applied in the detection field requiring the magnification of a slight displacement with linear spot position indicating.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a structure diagram of a device for detecting a liquid level of molten silicon of the present invention;
• FIG. 2 is a structure diagram of a CCD detecting device in the device for detecting a liquid level of molten silicon of the present invention;
• FIG. 3 is a structure diagram of a laser device in the device for detecting a liquid level of molten silicon of the present invention;
• FIG. 4 is a block diagram of a signal processing device in the device for detecting a liquid level of molten silicon of the present invention; and
• FIG. 5 is a schematic diagram of a linear magnification of a curved minor in the method of the present invention.
• In the figures:
• 1. liquid surface of molten silicon;2. laser beam;3. laser device;4. CCD detecting device;5. gold-plated quartz glass;6. single crystal furnace;7. crucible;8, crucible lifting device;9. optical filter;10. connector;11. curved mirror;12. CCD sensor;13. driving board;14, temperature control device;15. laser;16. electrical connector;17. cooling air port;18. control circuit board;19. power source;20. laser driving circuit;21. laser temperature control circuit;22. microprocessor a;23. CCD driving circuit;24. image signal processing circuit;25. RS485 bus;26. power line; and27. microprocessor b.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Hereinafter, the present invention is described in details in connection with the drawings and the specific embodiments.
• The present invention adopts a structure of a laser liquid surface detection device with linear optical magnification of a curved minor.FIG. 1 shows the mechanical portions of the laser liquid surface detection device, wherein acrucible7 is disposed within asingle crystal furnace6, acrucible lifting device8 is at a lower end of thecrucible7, openings are formed at the both sides of the top end of thesingle crystal furnace6 symmetrically, gold-platedquartz glasses5 are disposed at the two openings, alaser device3 and aCCD detecting device4 are disposed corresponding to the locations of the two openings, respectively, and signal processing devices are disposed within thelaser device3 and theCCD detecting device4.
• The structure of theCCD detecting device4 is shown inFIG. 2, wherein anoptical filter9 is horizontally disposed at a bottom of a housing of theCCD detecting device4, a drivingboard13 is vertically disposed at an inner wall of theCCD detecting device4, aCCD sensor12 is vertically disposed at an inner side of the drivingboard13, acurved minor11 having a concave reflective surface is also disposed within theCCD detecting device4, an emitting surface of thecurved mirror11 is disposed obliquely, facing theoptical filter9 and theCCD sensor12, and aconnector10, which is connected to external circuits, is disposed at a top of theCCD detecting device4.
• The structure of thelaser device3 is shown inFIG. 3, wherein alaser15 is disposed within a housing of thelaser device3, atemperature control device14 is disposed at an outer surface of thelaser15, thelaser15 and thetemperature control device14 are respectively connected to acontrol circuit board18, and anelectrical connector16, which is connected to external circuits, and a coolingair port17 are respectively disposed at a top of the housing of thelaser device3.
• The block diagram of the signal processing device in the device for detecting a liquid level of molten silicon using a laser reflected from a curved minor adopted by the present invention is shown inFIG. 4 as a signal processing portion, which comprises apower source19, wherein thepower source19 is connected to thelaser device3 and theCCD detecting device4 viapower lines26, respectively, anRS485 bus25 is also connected to thelaser device3 and theCCD detecting device4, respectively. Thelaser device3 includes alaser driving circuit20, a lasertemperature control circuit21, and a microprocessor a22, and theCCD detecting device4 includes aCCD driving circuit23, an imagesignal processing circuit24, and amicroprocessor b27. In particular, the microprocessor a22 and themicroprocessor b27 are respectively coupled to theRS485 bus25. Thelaser driving circuit20 is used to drive thelaser15, the lasertemperature control circuit21 is used to control thetemperature control device14, theCCD driving circuit23 is used to drive theCCD sensor12, and the image signal processing circuit is used to process the signals passing through the drivingboard13.
• The working principle of the device for detecting a liquid level of molten silicon using a laser reflected from a curved mirror adopted by the present invention is described as follows. Molten silicon liquid to be detected is disposed within thecrucible7, and thecrucible lifting device8 is used to adjust the up and down positions of thecrucible7 within thesingle crystal furnace6. Thelaser15 and thetemperature control device14 are controlled by thecontrol circuit board18. When the temperature of thelaser15 is excessively high, cool air passes through the coolingair port17, and then enters into thelaser device3 to lower the temperature. Thelaser15 emitslaser beam2, and thelaser beam2 passes through the gold-platedquartz glass5, and then is reflected by the liquid surface ofmolten silicon1, and finally enters into theCCD detecting device4, such that a basic Laser Triangulation Method is performed. Thelaser beam2 entered into theCCD detecting device4 is first filtered by theoptical filter9, so as to remove the light with different spectrum from the spectrum of the laser beam, preventing the interference on theCCD sensor12 of other light sources, and then thelaser beam2 is reflected by thecurved mirror11 to theCCD sensor12. The signals output by theCCD sensor12 are processed by the drivingboard13, and then transmitted through theconnector10 as detection signals. When the liquid surface ofmolten silicon1 is slightly changed, thelaser beam2 is reflected by the concave surface of thecurved minor11, and thus a large displacement occurs between the current incident position and the former incident position on theCCD sensor12, such that the detection accuracy is improved and the reading and writing of the signal is facilitated.
• The laser liquid surface detection device of the present invention is suitable for a liquid surface detection of molten silicon of a straight pulling type single crystal furnace. The laser liquid surface detection device of the present invention has a good function of linear magnification, which may magnify a slight change in the displacement of the liquid surface as a change in the displacement of a spot on the photodetector, resulting in high accuracy, short time for detection, low failure rate, easy installation and operation. The CCD detecting device of the laser liquid surface detection device of the present invention may also be used in general spot position indicating systems, so as to obtain a position indicating device with the function of linear magnification.
• In the present invention, the curved mirror is a cylindrical surface minor having a specific function relationship. The function relationship ensures that the corresponding relationship between the change in the liquid surface of molten silicon and the position obtained on the CCD sensor is linear. In order to obtain the linear corresponding relationship, the present invention provides a method for detecting a position of a liquid surface using the above laser liquid surface detection device with the specific steps as follows.
• Step 1
• Two openings are disposed at the top of thesingle crystal furnace6 symmetrically. Gold-platedquartz glasses5 are installed at the two openings. Thelaser device3 and theCCD detecting device4, in which thecurved mirror11 is not installed, are respectively disposed at the positions corresponding to the two openings. The signal processing devices are pre-disposed in thelaser device3 and theCCD detecting device4. The structures of thelaser device3, theCCD detecting device4, and the signal processing device are identical with those in the laser liquid surface detection device using the linear optical magnification of the curved mirror of the present invention.
• Step 2
• The shape of the reflective surface of thecurved minor11 and the position of thecurved minor11 within theCCD detecting device4 are determined as follows.
• First, the laser path formed by the laser incident from the incident point of thelaser device3 is mapped to a corresponding two-dimensional coordinate system. In the established coordinate system, the laser is reflected by an assumed curved minor11 after being reflected from the liquid surface of molten silicon, and the path of the laser is analyzed. The principle of linear magnification of the curved mirror is shown inFIG. 5.
• The laser is emitted from the laser incident point of thelaser15. A horizontal plane between the laser incident point A and the bottom of theCCD detecting device4 is set as the x-axis, the coordinate of the laser incident point is set as A(−m₀,0), and the angle formed by the point A and the x-axis is set as β. When the emitted laser reaches the lowermost end of the reflective surface of thecurved mirror11 after being reflected from the liquid surface of molten silicon liquid, the intersection point of the laser reflected from the liquid surface of molten silicon and the x-axis is set as A₁(m₀,0). The axis vertically passing through the midpoint of A(−m₀,0) and A₁(m₀,0) is set as the y-axis. At this time, the distance between the x-axis and the liquid surface of molten silicon is set as d₀, and the height of the liquid surface of the molten silicon is the maximum measurable value. The height of the liquid surface of molten silicon may decrease, and when the emitted laser reaches the uppermost end of the reflective surface of thecurved minor11 after being reflected from the liquid surface of molten silicon, the current height of the liquid surface of molten silicon is the minimum measurable value. The difference between the maximum value and the minimum value of the height of the liquid surface of molten silicon is the detectable range r. When the height of the liquid surface of molten silicon is changed from high to low, the reflecting points of the laser on the liquid surface of molten silicon are set as V₀(T₀,D₀), V₁(T₁,D₁), . . . , V_i(T_i,D_i) in sequence, and the lines formed by the reflection of the liquid surface of molten silicon are set as f₀, f₁, . . . , f_iin sequence. The curve function of the reflective surface of thecurved mirror11 is set as y=f(x). The reflecting points on thecurved minor11 are set as C₀(x₀,y₀), C₁(x₁,y₁), . . . , C_i(x_i,y_i) in sequence, the lines formed by the reflection of thecurved minor11 are set as f₀′, f₁′, . . . , f_i′ in sequence. The angles formed by f₀′, f₁′, . . . , f_i′ and the x-axis are set as α₀, α₁, . . . , α_iin sequence. The reflection tangent line of f_iand f_i′ at the point C_i(x_i,y_i) is set as l_i. The laser is finally incident on theCCD sensor12 which is disposed vertically. The line representing theCCD sensor12 is set as x=L+m₀. The incident points on theCCD sensor12 are set as P₀(L+m₀,Q₀), P₁(L+m₀,Q1), . . . , P_i(L+m₀,Q_i) in sequence.
• The coordinates of the laser incident points and the incident angle β are known, the detectable range r of the change in the liquid surface of molten silicon and the height range R of the reflected laser displayed on the CCD sensor are known, the magnification is N=R/r, and the position coordinates of respective incident points on the CCD sensor may be read directly, therefore, in the method for detecting a position of a liquid surface of the present invention, the changes in the height of the liquid surface of molten silicon may be detected only in the case that the curve function and the position of the reflective surface of the curved minor are given, a curved minor which satisfies the curve function of the reflective surface is then prepared, and the curved mirror is finally placed within the CCD detecting device. The numerical algorithm for the curve function of the reflective surface of the curved minor is given as follows.
• Calculation of the discrete point C_i(x_i,y_i) in the curve of the curved minor:
• First, l₀is obtained using f₀and f₀′, according to the optical reflection principle. Since V₀(T₀,D₀) on f₀and the angle β are already known, the function of f₀is known, and C₀(x₀,y₀) and P₀(L+m₀,Q₀) may be determined. The reflection tangent line l₀at the intersection point C₀(x₀,y₀) is obtained according to the incident line f₀and the reflected line f₀′. When the liquid surface moves downwards by a distance d, that is, the reflecting point of the liquid surface is changed to be V₁(T₁,D₁), it may be calculated that there shall be a point P₁(L+m₀,Q₁) on the CCD sensor. In this case, the intersection point of lines l₀and f₁is obtained as C₁(x₁,y₁), a line f₁′ is obtained using points C₁(x₁,y₁) and P₁(L+m₀,Q₁), and then the reflection tangent line l₁at the intersection point C₁(x₁,y₁) of lines f₁and f₁′ is obtained using lines f₁and f₁′, according to the optical reflection principle. At this time, C₁(x₁,y₁) is obtained through C₀(x₀,y₀), and in turns, using the iterative method, the intersection point of l_i-1and f_iis set as C_i(x_i,y_i), a line f_i′ is obtained using the points C_i(x_i,y_i) and P_i(L+m₀,Q_i), and then a line l_iis obtained using f_iand f_i′, according to the optical reflection principle, until all the discrete data points C_i(x_i,y_i) (wherein i=0, 1, 2, . . . , n) are obtained.
• Polynomial curve fitting is performed by using the obtained discrete data points C_i(x_i,y_i) as follows:
• An M-order polynomial curve fitting is performed using the discrete data points C_i(x_i,y_i) obtained by the above calculation, the fitted polynomial curve of the curved mirror is as follows:
• 
  y=f(x)=α_Mx^M+α_M-1x^M-1+ . . . +α₀
• Wherein x is a value selected from a range (x₀,x_n), M is a value selected from a range 3-5, and α₀, α₁, . . . , α_Mare the parameters given by the least squares curve fitting algorithm. The least squares curve fitting algorithm is a general mathematical algorithm, and therefore no further explanation is provided here.
• Step 3
• According to the polynomial curve function of the curved minor calculated instep 2, thecurved mirror11 is prepared, such that the radian function of the reflective surface of thecurved minor11 is consistent with the polynomial curve function of the curved mirror. Next, thecurved minor11 is disposed within theCCD detecting device4, such that the reflective surface of thecurved mirror11 is disposed obliquely, and the reflective surface faces theoptical filter9 and theCCD sensor12, the position of thecurved mirror11 within theCCD detecting device4 may be determined by the values defined by the polynomial curve function of the curved minor obtained as above.
• Step 4
• Thelaser device3 is activated, and the laser is emitted from the laser incident point of thelaser device3 towards the liquid surface of molten silicon. The laser enters into theCCD detecting device4 after being reflecting from the liquid surface of molten silicon, and therefore, the change in the height of the liquid surface of molten silicon may be detected by theCCD sensor12.
• The detailed detecting processes of the change in the height of the liquid surface of molten silicon are as follows. Since the maximum variable range r of the height of the molten silicon liquid surface is known and the maximum measureable range R of the spot on theCCD sensor12 is known, the magnification of thecurved minor11 may be obtain as N=R/r. The spot on theCCD sensor12 is changed as the liquid level of the liquid surface of molten silicon is changed. The changed value of the height of the spot on theCCD sensor12 is recorded, and then is divided by the magnification N, so that the changed value of the height of the liquid surface of molten silicon may be obtained.
Embodiments
• In the example of a laser triangulation liquid level detection system for a single crystal furnace, wherein the height parameter of the initial liquid surface is d₀=1300 mm, the respective parameters insteps 1 and 2 are set as follows, according to the above liquid surface position detection method: the laser incident point is set as m₀=300 mm, the height parameter of the initial liquid surface is set as d₀=1300 mm, the distance from the start point of the curved mirror to the CCD sensor is set as L=300 mm, the detectable range is set as r=40 mm, and the displayable range of the CCD sensor is set as R=75 mm. In order to facilitate the calculation, the curved minor C₀(x₀,y₀) is assumed as being on the x-axis, that is, C₀(m₀,0), and a iterative calculation is performed according to the calculation method of the discrete points C_i(x_i,y_i) of the curve of the curved mirror, wherein the displacement variation step is set as d=0.01 mm, and therefore, all the discrete data points C_i(x_i,y_i) (wherein i=0, 1, 2, . . . , n) may be calculated.
• An M-order polynomial curve fitting (wherein M=3) is performed using the discrete data points C_i(x_i,y_i) obtained by the above calculation, and the fitted polynomial curve function of the curved minor is as follows:
• $y=-1023.29960+6.30977x-0.01135x^2+0.0000056x^3x\in \left[300,323.14\right]$
• The linear magnification is N=1.875
• At last, a linear magnifying curved mirror having the above curve function may be prepared according tosteps 3 and 4, and the location to be disposed may be determined by said function. The changed height value displayed on the CCD sensor is divided by the linear magnification N, so as to obtain the changed value of the actual height of the liquid surface.

Claims (2)

1. A device for detecting a liquid level of molten silicon using a laser reflected from a curved mirror, which characterized in that the device comprises two openings disposed at a top of a single crystal furnace (6) symmetrically, gold-plated quartz glasses (5) disposed at the two openings, a laser device (3) and a CCD detecting device (4) respectively disposed corresponding to the two openings, and signal processing devices disposed within the laser device (3) and the CCD detecting device (4);

the CCD detecting device (4) has a structure comprising: a detecting device housing, an optical filter (9) horizontally disposed at a bottom of the detecting device housing, a driving board (13) vertically disposed at an inner wall of the detecting device housing, a CCD sensor (12) vertically disposed at an inner side of the driving board (13), a curved minor (11) having a concave reflective surface disposed within the detecting device housing, and a connector (10) disposed at a top of the detecting device housing, wherein the reflective surface of the curved minor (11) is disposed obliquely, the reflective surface faces the optical filter (9) and the CCD sensor (12);

the laser device (3) has a structure comprising: a laser device housing, a laser (15) disposed within the laser device housing, and a temperature control device (14) disposed at an outer surface of the laser (15), a control circuit board (18) to which the laser (15) and the temperature control device (14) are respectively connected , and an electrical connector (16) and a cooling air port (17) respectively disposed at a top of the laser device housing;

the signal processing device has a structure comprising: a laser driving circuit (20), a laser temperature control circuit (21), and a microprocessor a (22) disposed within the laser device (3), and a CCD driving circuit (23), an image signal processing circuit (24), and a microprocessor b (27) disposed within the laser device (4), wherein the laser device (3) and the CCD detecting device (4) are respectively connected to an RS485 bus (25).

2. A method for detecting a liquid level of molten silicon using a laser reflected from a curved minor, which characterized in that the method comprising steps of:

Step 1

disposing two openings at a top of a single crystal furnace (6) symmetrically, installing gold-plated quartz glasses (5) installed at the two openings, disposing a laser device (3) and a CCD detecting device (4) in which a curved minor (11) is not installed respectively disposed at positions corresponding to the two openings, wherein signal processing devices are pre-disposed within the laser device (3) and the CCD detecting device (4),

wherein the signal processing device has a structure comprising: a laser driving circuit (20), a laser temperature control circuit (21), and a microprocessor a (22) disposed within the laser device (3), and a CCD driving circuit (23), an image signal processing circuit (24), and a microprocessor b (27) disposed within the laser device (4), wherein the laser device (3) and the CCD detecting device (4) are respectively connected to an RS485 bus (25);

Step 2

determining a shape of a reflective surface of the curved mirror (11) and a position of the curved minor (11) within the CCD detecting device (4), which comprising:

a laser path formed by laser incident from an incident point of the laser device (3) is mapped to a corresponding two-dimensional coordinate system, it is assumed that the curved minor (11) is disposed within the CCD detecting device (4), a reflective surface of the curved minor (11) is disposed obliquely, the reflective surface faces an optical filer (9) and a CCD sensor (12), and the laser is reflected by the curved mirror (11) after being reflected from a liquid surface of molten silicon, in the established coordinate system, the laser path is analyzed, wherein the laser is emitted from a laser incident point, a horizontal plane between the laser incident point and a bottom of the CCD detecting device (4) is set as an x-axis, a coordinate of the laser incident point is set as A(−m₀,0), an angle formed by the laser incident point and the x-axis is set as β, when the emitted laser reaches a lowermost end of the reflective surface of the curved mirror (11) after being reflected from the liquid surface of molten silicon liquid, an intersection point of the laser reflected from the liquid surface of molten silicon and the x-axis is set as A₁(m₀,0), an axis vertically passing through a midpoint of A(−m₀,0) and A₁(m₀,0) is set as a y-axis, a current distance between the x-axis and the liquid surface of molten silicon is set as d₀, and a current height of the liquid surface of the molten silicon is a maximum measurable value; when the height of the liquid surface of molten silicon decreases and the emitted laser reaches an uppermost end of the reflective surface of the curved minor (11) after being reflected from the liquid surface of molten silicon, a current height of the liquid surface of molten silicon is a minimum measurable value, and a difference between the maximum value and the minimum value of the height of the liquid surface of molten silicon is the detectable range r; when the height of the liquid surface of molten silicon is changed from high to low, reflecting points of the laser on the liquid surface of molten silicon are set as V₀(T₀,D₀), V₁(T₁,D₁), . . . , V_i(T_i,D_i) in sequence and the lines formed by the reflection of the liquid surface of molten silicon are set as f₀, f₁, . . . , f_iin sequence, a curve function of the reflective surface of the curved minor (11) is set as y=f(x), reflecting points on the curved mirror (11) are set as C₀(x₀,y₀), C₁(x₁,y₁), . . . , C_i(x_i,y_i) in sequence, lines formed by the reflection of the curved minor (11) are set as f₀′, f₁′, . . . , f_i′ in sequence, angles formed by f₀′, f₁′, . . . , f_i′ and the x-axis are set as α₀, α₁, . . . , α_iin sequence, and a reflection tangent line of f_iand f_i′ at the point C_i(x_i,y_i) is set as l_i, the laser is finally incident to the CCD sensor (12) which is disposed vertically, a line representing the CCD sensor (12) is x=L+m₀, and incident points on the CCD sensor (12) are set as P₀(L+m₀,Q₀), P₁(L+m₀,Q₁), . . . , P_i(L+m₀,Q_i) in sequence,

calculating discrete points C_i(x_i,y_i) on the curve of the curved mirror, which comprising

first, l₀is obtained using f₀and f₀′ according to the optical reflection principle, wherein V₀(T₀,D₀) on f₀and an angle β are known, therefore the function of f₀is known, and C₀(x₀,y₀) and P₀(L+m₀,Q₀) are determined; the reflection tangent line l₀at the intersection point C₀(x₀,y₀) is obtained according to the incident line f₀and the reflected line f₀′; C₁(x₁,y₁) is obtained through C₀(x₀,y₀) by calculating the point P₁(L+m₀,Q₁) on the CCD sensor when the liquid surface moves downwards by a distance d, that is, the reflecting point of the liquid surface is changed to be V₁(T₁,D₁), the point P₁(L+m₀,Q₁) on the CCD sensor is determined, the intersection point C₁(x₁,y₁) of l₀and f₁is obtained, the line f₁′ is obtained using the points C₁(x₁,y₁) and P₁(L+m₀,Q1), and then the reflection tangent line h at the intersection point C₁(x₁,y₁) of lines f₁and f₁′ is obtained using lines f₁and f₁′ according to the optical reflection principle, so that C₁(x₁,y₁) is calculated based on C₀(x₀,y₀); the line f₁′ is obtained using the points C_i(x_i,y_i), which is an intersection point of l_i-1and f_i, and P_i(L+m₀,Q_i), and then the line l_iis obtained using f_iand f_i′ according to the optical reflection principle, using iterative method until all the discrete data points C_i(x_i,y_i) are obtained, wherein i=0, 1, 2, . . . , n,

polynomial curve fitting using the obtained discrete data points C_i(x_i,y_i), which comprising

an M-order polynomial curve fitting is performed using the discrete data points C_i(x_i, y_i) obtained by the above calculation, wherein the fitted polynomial curve of the curved mirror is as follows:

y=f(x)=α_Mx^M+α_M-1x^M+ . . . +α₀

wherein x is a value selected from a range (x₀,x_n), M is a value selected from a range 3-5, and α₀, α₁, . . . , α_Mare the parameters given by the least squares curve fitting algorithm;

Step 3

preparing the curved mirror (11) according to the polynomial curve function of the curved minor calculated in step 2, such that a radian function of the reflective surface of the curved minor (11) is consistent with the polynomial curve function of the curved minor, next, disposing the curved minor (11) within the CCD detecting device (4), such that the reflective surface of the curved mirror (11) is disposed obliquely, and the reflective surface faces the optical filter (9) and the CCD sensor (12), wherein the position of the curved minor (11) within the CCD detecting device (4) is determined by the values defined by the polynomial curve function of the curved minor obtained as above;

Step 4

detecting a change in the height of the liquid surface of molten silicon, which comprising: activating the laser device (3), emitting the laser from the laser incident point of the laser device (3) towards the liquid surface of molten silicon, detecting the change in the height of the liquid surface of molten silicon by the CCD sensor (12) when the laser enters into the CCD detecting device (4) after being reflecting from the liquid surface of molten silicon, wherein the maximum variable range r of the height of the molten silicon liquid surface is known and the maximum measureable range R of a spot on the CCD sensor (12) is known, therefore, the magnification of the curved minor11 is obtained as N=R/r, the spot on the CCD sensor (12) is changed as the liquid level of the liquid surface of molten silicon is changed, the changed value of the height of the spot on the CCD sensor (12) is recorded, and then is divided by the magnification N, and therefore, the changed value of the height of the liquid surface of molten silicon is obtained.

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